This invention relates to a process and apparatus for reducing the nitrogen oxide content in oxygen-containing emissions of internal-combustion engines, particularly of diesel engines and directly injecting Otto engines for motor vehicles, where the nitrogen oxides are reduced on a catalyst by means of a reducing agent added in a metered manner to engine exhaust gases.
Such a process is known from European Patent Document EP 537,968 Al, in which case the nitrogen oxides are converted by means of hydrogen as a reducing agent on a platinum/zeolite catalyst. However, the catalytic NOx reduction on platinum-containing catalysts has the disadvantage that a sufficient conversion rate is ensured only in a relatively narrow temperature window.
Furthermore, it is known to reduce nitrogen oxides by means of hydrocarbons as reducing agents by means of a so-called DENOX catalyst. Such processes have their conversion maximum at higher temperatures. However, in the European driving cycle required by law, even in the case of the selection of optimal conditions concerning the reducing agent and the DENOX-catalyst, so far an NOx-reduction of only 10-15 has been achieved. This, however, it not sufficient for ensuring future emission limit values for motor vehicles. In addition, in the case of modern diesel engines, there is the problem that hydrocarbons are not longer contained to a sufficient extent in the emissions.
It is an object of the invention to improve a process for reducing the nitrogen oxide content in oxygen-containing emissions such that an increased conversion rate is ensured over a wide temperature range, including at temperatures starting from approximately 100.degree. C.
According to the invention, this object is achieved by a process of the above mentioned type comprising operating the engine in a first operating range with only hydrogen being supplied as a reducing agent to the exhaust gas upstream of the catalyst, operating the engine in a second operating range with both hydrogen and hydrocarbons being supplied as reducing agents to the exhaust gas upstream of the catalyst, and operating the engine in a third operating range with only hydrocarbons being supplied as a reducing agent to the exhaust gas upstream of the catalyst.
The invention is based on the experimentally confirmed recognition that, for ensuring a sufficient NOx-reduction, hydrogen as well as hydrocarbons must be contained in the exhaust gas at a ratio predetermined as a function of operating conditions. As a result of the suitable combination of both reducing agents, a synergistic effect is obtained, in which case the hydrogen has an activating effect on the course of the NOx-reduction. In this case, the hydrogen increases the selectivity of the HC-NOx reaction and simultaneously reduces the selectivity for the production of nitrous oxide (N.sub.2 O).
During the tests carried out on platinum-containing catalysts, it was found that, in the presence of a minimum concentration of hydrocarbons in the exhaust gas, in addition to the conversion maximum of approximately 100.degree. C. characteristic of hydrogen, a second conversion maximum between 250.degree. and 350.degree. C. can be observed. In addition, also at very low hydrocarbon Nox ratios at which so far only low NOx conversions could be observed, relatively high NOx conversion rates were achieved in the presence of small amounts of hydrogen.
The decisive factor for a sufficient NOx reduction on a DENOX catalyst is therefore the correct proportion of hydrogen to hydrocarbons in the exhaust gas as a function of the catalyst temperature. For this reason, the proportioning of the two reducing agents preferably takes place as a function of the catalyst temperature and/or the engine load and/or the rotational engine speed, preferably controlled by a characteristic diagram. The physical quantity on which the proportioning is based is the NOx flow rate.
Many processes can be used for providing hydrocarbons. In the case of engines which are equipped with a so-called commonrail high-pressure injection system, the possibility is particularly advantageous of adjusting a desired proportion of hydrocarbons in the exhaust gas by a targeted afterinjection of a given fuel quantity into the internal-combustion engine. In the case of such engines, in addition, by means of an early afterinjection of fuel, it is possible to produce the required hydrogen.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.